1. Field of the Invention
The present invention relates to control valves, and in particular, to a control valve for a variable displacement gas compressor for use in an air conditioning or refrigeration system
2. Description of the Related Art
A gas compressor will change a state of a gas from a low-pressure state to a high-pressure state. Such a compressor is often used in air-conditioning (A/C) systems where expansion of a refrigerant gas compressed by the compressor causes air passing over evaporator gas tubes to cool. After the gas has expanded, it is recycled through the compressor so to be compressed again.
The refrigerant gas is discharged by the compressor at a high pressure known as the discharge pressure. It moves to a condenser, where the high pressure, high temperature gas condenses into a high pressure, high temperature liquid, the energy required for the state change being transferred to air passing over the condenser fins in the form of heat. From the condenser, the liquid travels through an expansion device, where it expands, to an evaporator where it evaporates. The air passing over the evaporator coils gives off its heat to the refrigerant, providing energy needed for the state change. The cooled air passes out into the compartment to be cooled. The degree to which the air is cooled is proportional to the amount of expansion of the refrigerant gas, and the amount of expansion of the gas is directly proportional to how much gas is compressed within the compressor. The pressure of the gas is controlled within the compressor by the amount of displacement of the piston within the compression chamber.
A key concern in designing a cooling system utilizing refrigerant gas is too ensure that the liquid from the condenser does not flow in a quantity and temperature to push the evaporator below the freezing point of water. If there is too much heat absorption by the gas within the evaporator, the water found on the fins and tubes through condensation of water from air passing over the evaporator will freeze up, choking off air flow over the evaporator, thereby cutting off the flow of cool air to the passenger compartment. For this reason, most conventional control valves are calibrated to change the stroke (displacement) of the compressor based on the pressure of the gas returning to the compressor at a set pressure of the gas. The gas returns to the suction area of the compressor. The pressure in this area of the compressor is known as the suction pressure. The desired suction pressure, around which the stroke of the compressor is changed, is known within the art as the set-point suction pressure.
In 1984, a variable displacement refrigerant compressor was introduced which adjusted the flow of the refrigerant gas through the system by varying the stroke of the piston in the pumping mechanism of the compressor in the manner just described. This system was designed for use in an automobile, deriving power to drive the compressor using a drive belt coupled to the vehicle""s engine. In operation, when the A/C system load is low, the piston stroke of the compressor is shortened so that the compressor pumps less refrigerant per revolution of the engine drive belt. This allows just enough refrigerant to satisfy the cooling demands of the automobile""s occupants. When the A/C system load is high, the piston stroke is lengthened and pumps more refrigerant per revolution of the engine drive belt.
A description of this prior art variable displacement compressor and a conventional pneumatic control valve (CV) is found in U.S. Pat. No. 4,428,718 by Skinner (Skinner ""718) which is assigned to the General Motors Corporation of Detroit, Mich. The Skinner ""718 description and explanation of the variable displacement compressor, general function, and interaction of the CV with the compressor is hereby incorporated by reference.
FIG. 9 shows a variable displacement refrigerant compressor as described by Skinner ""718. There is shown a variable displacement refrigerant compressor 210 of the variable angle wobble plate type connected in an automotive air conditioning system having the normal condenser 212, orifice tube 214, evaporator 216 and accumulator 218 arranged in that order between the compressor""s discharge and suction sides. The compressor 210 comprises a cylinder block 220 having a head 222 and a crankcase 224 sealingly clamped to opposite ends thereof. A drive shaft 226 is supported centrally in the compressor at the cylinder block 220 and crankcase 224 by bearings. The drive shaft 226 extends through the crankcase 224 for connection to an automotive engine (not shown) by an electromagnetic clutch 236 which is mounted on the crankcase 224 and is driven from the engine by a belt 238 engaging a pulley 240 on the clutch 236.
The cylinder block 220 has five axial cylinders 242 through it (only one being shown), which are equally spaced about and away from the axis of drive shaft 226. The cylinders 242 extend parallel to the drive shaft 226 and a piston 244 is mounted for reciprocal sliding movement in each of the cylinders 242. A separate piston rod 248 connects the backside of each piston 244 to a non-rotary, ring-shaped, wobble plate 250.
The non-rotary wobble plate 250 is mounted at its inner diameter 264 on a journal 266 of a rotary drive plate 268. The drive plate 268 is pivotally connected at its journal 266 by a pair of pivot pins (not shown) to a sleeve 276 which is slidably mounted on the drive shaft 226, to permit angulation of the drive plate 268 and wobble plate 250 relative to the drive shaft 226. The drive shaft 226 is drivingly connected to the drive plate 268. The wobble plate 250 while being angularable with the rotary drive plate 268 is prevented firm rotating therewith by a guide pin 270.
The angle of the wobble plate 250 is varied with respect to the axis of the drive shaft 226 between the solid line large angle position shown in FIG. 9, which is full-stroke, to the zero angle phantom-line position shown, which is zero stroke, to thereby infinitely vary the stroke of the pistons and thus the displacement or capacity of the compressor between these extremes. There is provided a split ring return spring 272 which is mounted in a groove on the drive shaft 226 and has one end that is engaged by the sleeve 276 during movement to the zero wobble angle position and is thereby conditioned to initiate return movement.
The working ends of the cylinders 242 are covered by a valve plate assembly 280, which is comprised of a suction valve disk and a discharge valve disk, clamped to the cylinder block 220 between the latter and the head 222. The head 222 is provided with a suction area 282 which is connected through an external port 284 to receive gaseous refrigerant from the accumulator 218 downstream of the evaporator 216. The suction area 282 is open to an intake port 286 in the valve plate assembly 280 at the working end of each of the cylinders 242 where the refrigerant is admitted to the respective cylinders on their suction stroke each through a reed valve formed integral with the suction valve disk at these locations. Then on the compression stroke, a discharge port 288 open to the working end of each cylinder 242 allows the compressed refrigerant to be discharged into a discharge area 290 in the head 222 by a discharge reed valve which is formed integral with the discharge valve disk. The compressor""s discharge area 290 is connected to deliver the compressed gaseous refrigerant to the condenser 212 from whence it is delivered through the orifice tube 214 back to the evaporator 216 to complete the refrigerant circuit as shown in FIG.9.
The wobble plate angle and thus compressor displacement can be controlled by controlling the refrigerant gas pressure in the sealed interior 278 of the crankcase behind the pistons 244 relative to the suction pressure. In this type of control, the angle of the wobble plate 250 is determined by a force balance on the pistons 244 wherein a slight elevation of the crankcase-suction pressure differential above a suction pressure control set-point creates a net force on the pistons 244 that results in a turning moment about the wobble plate pivot pins (not shown) that acts to reduce the wobble plate angle and thereby reduce the compressor capacity.
An important element of the variable displacement compressor is a pneumatic control valve 300 inserted into the head portion 222 of the compressor. CV 300 senses the A/C load by sensing the pressure state (the suction pressure) of the refrigerant gas returning to the compressor. The CV is operably connected to the crankcase chamber 278. There are channels in the cylinder block 220 and the head 222 of the compressor for gas flow between the CV and suction area 282, discharge area 290 and crankcase chamber 278 of the compressor. The CV controls the displacement of a piston 244 within the compressor by controlling the pressure of gas in the crankcase chamber 278 that acts on the backside of the pistons 244 and the wobble plate 250.
Control valve 300 inserts into a stepped, blind CV cavity 298 formed in the compressor head 222. The blind end of CV cavity 298 communicates directly with discharge area 290 through port 292. CV cavity ports 294 and 295 communicate with the crankcase chamber 278. CV cavity port 296 communicates with the suction area 282. CV 300 is sealed into the CV cavity 298 so that particular features of the CV align with ports 292, 294, 295 and 296.
FIG. 10 illustrates, in more detail, the pneumatic CV 300 depicted in FIG. 9. The valve 300 comprises a valve body 301 and valve bellows cover 312. Grooves 314, 316 and 318 are formed in the valve body to position o-rings which seal against the walls of the CV cavity 298. A groove 299 formed in the wall of the CV cavity 298 holds an o-ring which seals against the valve bellows cover 312. This arrangement of o-rings seals the valve into four regions within the CV cavity 298 that are sealed with respect to each other and are each in gas communication with one of ports 292, 294, 295 or 296.
CV 300 has an upper valve chamber 330 that communicates to the compressor discharge area 290 via (through) filter 320 and CV cavity port 292. A mid-valve chamber 322 communicates to the crankcase chamber 278 via an opening 321 in the valve body 310. A central passageway 326 in the valve body 310 communicates with the crankcase chamber 278 via port 295. A lower valve chamber 328 communicates with the compressor suction area 282 through opening 327 in the valve bellows cover 312 and via port 296.
CV 300 has a ball valve comprising ball 332 and valve seat 334 that can be operated to control the flow communication path between upper valve chamber 330 and mid-valve chamber 322, hence controlling the flow communication between the discharge area 290 and the crankcase chamber 278 of the compressor. CV 300 has a conical valve consisting of conical member 340 and matching conical valve seat 338 that can be operated to control the flow communication between lower valve chamber 328 and central passageway 326, hence controlling the flow communication between the suction area 282 of the compressor and the crankcase chamber 278.
The conical valve member 340 is formed as a shoulder near one end of a valve rod 336. The other end of valve rod 336 is arranged to push against ball 332 as the conical valve member 340 is seated against the matching conical valve seat 338. With this arrangement, the movement of the valve rod 336 opens and closes the flow communication of both discharge pressure and suction pressure gas to the crankcase chamber 278. The positioning of valve rod 336 can be used to adjust the crankcase pressure to values between suction pressure and discharge pressure. This adjustment of the crankcase pressure, in turn, adjusts the compressor displacement.
In conventional pneumatic CV 300, the position of valve rod 336 is established by a balance of forces arising from the discharge pressure acting on ball 332, a pressure sensitive bellows actuator 350, ball centering spring 354 and bias spring 352. Bellows actuator 350 is comprised of an evacuated metal bellows 342, an internal spring 344, end caps 345 and 346, and bellows stem 348. The bellows actuator 350 is extended by the force of internal spring 344 and is contracted by the force of gas pressure applied to the external surface of the bellows. Bellows actuator 350 is sealed in lower valve chamber 328 that is in gas communication with the suction area 282 of the compressor.
During operation of the compressor, CV 300 responds to changes in the suction pressure of the compressor 210 via the bellows actuator 350, and to changes in the discharge pressure via the force on ball 332. The spring constants and nominal compression of the bellows internal spring 344, bias spring 352 and ball centering spring 354 create forces on valve rod 336 that are set by the valve manufacturer at the time of valve assembly. The spring forces act to normally condition control valve 300 so as to open the flow of discharge pressure gas and simultaneously to close the flow to the suction area 282 from the crankcase chamber 278. CV 300 will therefore control the flow of discharge and suction pressure gasses to the compressor crankcase 278 according to these fixed spring forces.
The nominal spring bias force set-up design parameters in a pneumatic CV such as CV 300 are chosen so that during operation of the air conditioning system, the temperature of the evaporator is maintained slightly above the freezing point of water. The spring bias set-up requires a balancing of system objectives that apply under different air temperature ambient conditions. For higher air temperature ambient conditions, it is optimal to maintain as cold an evaporator as possible without freezing. At lower ambient air temperatures it is desirable to maintain as high an evaporator temperature as can be maintained while still supplying some dehumidification. One choice of spring bias forces for CV 300 must accommodate to multiple ambient air temperature conditions, engine power loading conditions, and user demands for cooling.
Pneumatic CV""s with fixed spring force bias set-up designs have two major disadvantages. First, the system is always working at its maximum capacity at the evaporator requiring maximum energy use by the compressor when the cooling system is operating. Second, since the evaporator is always at maximum capacity, hot air must be introduced into the system to temper the cold air to a temperature other than full cold.
An alternate CV design used in variable displacement compressors for vehicle air conditioning system utilizes a solenoid-assisted valve to control the flow of refrigerant gas into the crankcase of a variable displacement compressor. U.S. Pat. No. 5,964,578 by Suitou, et al (Suitou ""578), discloses a CV having a solenoid-activated rod that operates on a valve member that controls the flow of discharge and suction pressure gasses to the crankcase. The valve member position is partially established by a spring-biased bellows in similar fashion to a conventional pneumatic CV. Increasing suction pressure acts on the bellows to reduce gas flow from the discharge area to the crankcase. When energized, the solenoid activated rod applies a force that also urges the valve member so as to reduce discharge pressure flow to the crankcase. This allows an additional control of the piston stroke and the output capacity of the compressor that can be mediated by electrical signals to the solenoid coils.
An alternate CV design using a solenoid actuator to assist discharge valve operation has been disclosed in U.S. Pat. No. 5,702,235 by Hirota (Hirota ""235). In this design a solenoid is used to open and close a pilot valve that admits discharge pressure gas to a pressurizing chamber in the CV. The pressurizing chamber is in constant gas communication with the suction pressure area of the compressor. A valve member controls the flow of discharge and suction pressure gasses to the crankcase. The position of the valve member is established by a balance of spring bias forces, the force of the discharge pressure acting on an end of the valve member, and the force of the pressure in the pressurizing chamber acting on the opposite end of the valve member. When energized, the solenoid activated pilot valve allows the pressure to rapidly increase in the pressurizing chamber, opening the valve member to increase the flow of discharge pressure gas to the crankcase.
The valve member of the Hirota ""235 CV design does not respond to the suction area pressure and does not control compressor displacement according to a suction pressure set-point as does the solenoid-assisted CV of Suitou ""578 or the pneumatic CV of Skinner ""718. The object of the Hirota ""235 CV design is to use the force of discharge pressure gas to open the discharge to crankcase valve, thereby allowing the use of a compact, lightweight and inexpensive solenoid.
There are several major disadvantages with the prior art solenoid-assisted CV""s. First, a variable position solenoid is required. Variable position solenoids are not linear in performance and the extreme temperatures in an automobile engine compartment make proper operation of the variable position solenoid highly difficult given power constraints. Second, a large and precise current value is required to properly position the solenoid. Third, variable position solenoid systems do not provide a steady suction pressure set-point whereby the cooling system can maintain itself in a state of equlibrium.
As a solution to the inefficiencies of conventional pneumatic and solenoid-assisted CV""s, a CV design is needed in which the set-up of the bias forces acting within a pneumatic valve control valve can be changed to optimize the performance of the cooling system under different conditions. That is, a variable set-point control valve (VCV) is needed which varies the degree of displacement of the piston in the compression chamber. The suction pressure set-point is varied by the VCV according to the temperature desired by the occupants of the passenger compartment. In this manner, the cooling system does not have to operate at its maximum at all times, but rather the compressor only compresses and pumps enough the refrigerant gas to the suction pressure set-point necessary to cool the air flow to the temperature defined by the occupants. Substantial energy is saved by pressurizing the gas only to the point required and pumping only the volume required, and efficiencies are realized by eliminating the introduction of hot air into the cooled air flow.
A variable set-point CV is needed which overcomes the drawbacks of conventional pneumatic and solenoid-assisted CV""s and enables a cooling system that maintains a steady-state equilibrium to match the needs of the passengers in the passenger compartment while operating efficiently.
Accordingly, it is an objective of the present invention to provide a control method and a control valve used in variable displacement compressors, which valve maintains the pressure in the compressor crankcase in response to the suction pressure of the compressor relative to a stable, predetermined set-point of the suction pressure, which set-point can be changed during compressor operation by electrical signals.
To achieve the above objective the present invention discloses a control valve in a variable displacement compressor having a piston having a variable displacement within a compression chamber. A gas is admitted to the compression chamber from a suction area of the compressor at a suction pressure and discharged to a discharge area of the compressor at a discharge pressure. A gas pressure in a crankcase chamber acts upon the piston or mechanical elements linked to the piston, so that the displacement of the piston varies according to the crankcase pressure relative to the suction pressure. The control valve controls the crankcase pressure by means of a discharge valve portion that opens a gas communication path between the discharge area and the crankcase chamber. The discharge valve portion is operably coupled to a pressure sensitive member. The pressure sensitive member has a suction pressure receiving area in gas communication with the suction pressure area and a reference pressure receiving area in gas communication with a reference chamber. The reference chamber has a reference pressure established to a predetermined reference pressure by a flow of discharge and suction pressure gas to and from the reference chamber. The pressure sensitive member moves in response to the predetermined reference pressure and suction pressure changes to open the discharge valve portion. The control valve has reference chamber valve means for controlling the flow of discharge and suction pressure gas to and from the reference chamber in response to electrical signals, thereby establishing the predetermined reference pressure. The control valve of the invention is therefore capable of operating as a variable set-point control valve, wherein a stable set-point can be changed in response to electrical signals.
The present invention also discloses a variable set-point control valve for a variable displacement compressor that additionally controls the flow of suction pressure gas to the crankcase chamber by means of a suction pressure valve portion that opens or closes a gas communication path between the suction area and the crankcase chamber.
The present invention further discloses a method of controlling a variable displacement compressor having a piston having a displacement within a compression chamber, the compression chamber admitting gas at suction pressure and discharging gas at discharge pressure, the displacement of the piston varying according to the compressor crankcase pressure, and a control valve having a gas-filled reference chamber for controlling the crankcase pressure. The method comprises determining an amount of gas to be compressed to cause a condition to occur. A predetermined reference pressure within the reference chamber which will cause a crankcase pressure condition to occur is then determined. The reference pressure within the reference chamber is measured. An amount of discharge pressure gas flow to and suction pressure gas flow to the reference chamber, based on the predetermined reference pressure and the measured reference pressure, is calculated. At least one actuator to operate at least one reference chamber valve to cause the reference pressure to change towards the predetermined reference pressure by allowing the flow of discharge pressure and suction pressure gas into and out of the reference chamber is then actuated. A pressure sensitive member is moved according to the pressure within the reference chamber, the pressure sensitive member opening a gas communication path between the discharge area and the crankcase chamber.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description and drawings of the preferred embodiments of the present invention, and the claims.